The smallest inclination capable of maintaining the mobility of water is 1-1000000, but it is barely perceptible at twice that inclination. At 1-9288,the mean velocity is six inches per second ; at 1-2700, seven inches per second. The aqueducts of the ancients were inclined from 1-432 to 1-648. The minimum velocity necessary to maintain the salubrity of water is 13 3-4 English inches per second.